Fuel cells hold great promise for commercial use in mobile and stationary power supply systems. Fuel cells electrochemically convert fuels and oxidants to electricity. Fuel cell types include Alkaline Fuel Cells (AFC), Molten Carbonate Fuel Cells (MCFC), Phosphoric Acid Fuel Cells (PAFC), Proton Exchange Membrane Fuel Cells (PEMFC or PEM), Solid Oxide Fuel Cells (SOFC) and Direct Methanol Fuel Cells.
There has been significant progress in the development of fuel cells, including improvements in specific characteristics, such as increased power density and increased efficiency. Nonetheless, the wide variations in load demand encountered in most commercial applications remain a problem for fuel cell based electrochemical generators, particularly for those that use solid polymer electrolytes, such as PEMs.
A PEM fuel cell converts the chemical energy of fuels such as hydrogen and an oxygen containing gas (e.g. air) directly into electrical energy, water and heat. At the heart of a PEM fuel cell is a membrane electrode assembly (MEA) comprising a proton conducting membrane electrolyte sandwiched between two gas diffusion electrodes. The membrane permits the passage of protons (H+) generated by oxidation of hydrogen gas at the anode to reach the cathode side of the fuel cell and form water, while preventing passage therethrough of either of the reactant gases.
Efficient operation of PEM fuel cells generally requires the removal of a portion of the water produced. Excess water can dilute the electrolyte, making it difficult to maintain optimum electrolyte concentrations under wide ranging of current loads. Load demands faced by a system in a typical commercial use might vary from 0 to 1000 ma/cm2 under a typical load cycle.
For the optimum operation of such fuel cells, the membrane should remain sufficiently moist throughout, but not too moist. Thus, there must be removal of a portion of the water generated at the cathode, as well as the addition of water at the anode side to provide sufficient membrane moistness.
Several characteristics of PEM fuel cells separate them from other types of fuel cells. For example, in contrast to other fuel cell types, PEM fuel cells have a narrow range for controlling optimal concentration of electrolyte in the localized zone of electrochemical activity comprising the anode, membrane and cathode. Such membranes have a limited ability for redistribution of water over the fuel cell working surface area. This performance characteristic of fuel cells with PEMs is attributed to the reduced ability of the anode, cathode and membrane (as a group) to transport water, and to the hydrophobic characteristics of the materials used.
These characteristics of solid-polymer membranes become critical when designing and using fuel cells with large working surface areas to produce large currents, such as required for transportation applications (e.g. automobiles, and busses), especially when a large number of fuel cells are combined in series to generate high voltage outputs. For example, to build an electrochemical generator having a capacity of 25 kW at a voltage of 120V, a stack comprising 160 fuel cells is required with a working surface area of approximately 600 cm2 each. In a generator with a power rating of 60 kW and a 330V output, it is necessary to install 420 fuel cell elements with a working surface area of 740 cm2 each, connected in series.
Maintaining the high output characteristics of fuel cells assembled into stacks to form electrochemical generators is one of the challenges of electrochemical generator design. In the case of fuel cells with solid-polymer membranes this task is even more difficult. The very narrow range over which water concentration must be controlled imposes strict requirements on the systems that feed the working gases, as well as on regulation of water concentration and temperature of each individual fuel cell. In addition, even at low operating times (1000–2000 hrs), characteristics of the individual fuel cells in a stack do not change in a constant or even manner. Progressive and uneven degradation in performance among the cells demands even more strict requirements for control of fuel cells assembled into electrochemical generator systems.
In high power hydrogen-air electrochemical generators, hydrogen is supplied from storage tanks with high pressures up to 70 MPa. Systems for supplying gas usually have electric valves on hydrogen supply and purge lines. A hydrogen pressure regulator is commonly installed in the gas supply line upstream of the fuel cell stack. A feedback control pressure regulator is generally provided which senses variation in pressure at the fuel cell and control reactants gas flow in a manner proportional to gas usage. Control of gas flow and pressure (i.e. reduction of pressure from input pressure to working pressure) is also accomplished using a regulator.
For smoother and more precise throttle control, a two-stage pressure regulator system is usually installed. The pressure regulator reduces the working pressure of the fuel cell. For synchronization of hydrogen and air pressures in the fuel cell stack, a pressure reference line is installed in parallel to hydrogen supply line to provide a reference pressure to the regulator.
This reference line is static and does not consume hydrogen during fuel cell operation. It is filled with hydrogen during start-up and emptied (purged) when the fuel cell generator is stopped or stored. As a rule, a vent valve is installed in the reference line to restrict pressure, and an electrical valve is installed for reduction of pressure to atmospheric pressure.
The reference line can be filled with inert gas, if available. The oxidant feed line to the cathode pores in the fuel cell stack has a filter to remove particles and a compressor to built up air pressure to a working level. The partial pressure of oxygen in air is relatively low (about 21.6%), the largest portion of air being nitrogen. For the cathode to work effectively, air should be fed in excess. In this case, the efficiency of oxygen usage is 40%–60% as a rule. At higher rates of oxygen usage, the cathode is less efficient.
In current fuel cell stack designs, the air supply system maintains the design working pressure level on cathode and anode. For this purpose, the hydrogen pressure regulator has a feedback connection to the air supply line at the entry point to the fuel cell. In this case the hydrogen pressure in the anode chamber is constantly compared with the air pressure in the cathode chamber and the pressure regulator makes needed adjustments in order to maintain the correct pressure ratio.
The system described above for supplying hydrogen and air to fuel cells with solid-polymer electrolytes is essentially universal and used in almost all known designs with only minor variations. However, as explained below, these systems do not provide good regulation of the water concentration along the cathode and anode surface of the fuel cell stack, particularly for high and highly variable load conditions.
The power output of a hydrogen-air fuel cell mainly depends on effective performance of the cathodes (oxygen limited electrodes). At higher coefficient efficiency oxide-oxygen (CEUO), such as CEUO≧70%, stable fuel cell performance is generally not possible with current density j≧0.5 a/cm2 because of low oxygen concentrations in air near the exhaust point from cathode chamber.
In this case, there are gas transport restrictions on the amount of oxygen penetrating through the cathode pores and available to the cathodes. Drying takes place in some areas of the cathodes because of low water (vapor) concentration in the air supplied by the compressor.
Moreover, compressed feed air at the outlet of the compressor can be at even higher temperatures (e.g. 130–170° C.). Thus, there is active removal of water (vapor) by the airflow which, in turn, leads to drying of the membrane in the air inlet region. In the air outlet area from the cathodes there occurs the reverse of this process leading to “flooding” of the cathode and membrane because air flowing in this area close to saturation and the rate of water uptake (vaporization) is lower.
Because of low oxygen concentrations in the air after passing through most of the cathode chamber and gas flow restrictions, a large portion of the cathode surface can be in a condition of “concentration polarization.” Concentration polarization results from restrictions to the transport of the fuel gases to the reaction sites. This usually occurs at high current because the forming of product water and excess humidification blocks the reaction sites. In this situation, there is increased risk of cross polarization in area near the gas outlet from the cathode chamber. This risk becomes much greater when the fuel cell load is highly variable over short time periods. Specifically, the risk is greatest when loads are switched from low to high levels and back in short periods of time, such as tens of seconds to minutes.
Such short-term load variations are generally not allowed in fuel cell operation. Otherwise non-optimum water concentration at the cathode and membrane can lead to cross polarization. This can cause the cells to operate in an electrolysis mode, which in turn can lead to direct reaction between hydrogen and air in the cell resulting in physical damage to the fuel cell.
Solving the problem of controlling water concentrations in fuel cells will greatly expand their potential application. However, this does not solve the problem of the fuel cell's inability to withstand wide range, short-term variations in load because of high thermal inertia due to the heat capacity of the fuel cell stack and the heat exchanges. The primary unmet requirement for use of hydrogen-air fuel cells in transportation and many stationary power applications is that fuel cell generators must be highly reliable in the face of rapid and wide-range variations in load.
The above-mentioned issues represent a significant problem for electrochemical generators with solid polymer fuel cells as presently installed on electric vehicle prototypes. Currently available electrochemical generators do not meet consumer requirements in this regard, and therefore cannot be mass-produced and marketed for general use. This is not only because of the high cost and complexity of systems for controlling processes in fuel cells, but also because a primary application requirement cannot be met. This requirement is the ability to handle current loads that vary widely, and sometimes rapidly, for long-term operation (e.g. more than about 3000 hrs).